The present invention relates to a process of coupling at least two optical fibers by means of a holographic coupling lens, in which a recorded phase and volume hologram forms the holographic coupling lens.
It likewise relates to an apparatus for carrying out this process, comprising a rigid support arranged to keep the holographic coupling lens and the ends of the fibers to be coupled in given relative positions.
Coupling processes of this type, as well as apparatus for carrying them out, have already been described, in particular in U.S. Pat. No. 4,057,319 and in U.S. Pat. No. 4,310,216.
In U.S. Pat. No. 4,057,319, the ends of the fibers are respectively mounted in two complementary parts of a support, these parts being able to be assembled to constitute the support and to maintain the holographic coupling lens and the ends of the fibers to be coupled in a given relative position.
In U.S. Pat. No. 4,310,216 mentioned above, the coupling makes use of an "in situ" technique. The two fiber ends to be coupled are buried in a transparent block, within which is formed a first hologram, termed a provisional hologram, used for recording a second hologram, termed a definitive hologram, the latter of which is utilized as the coupling hologram for the two optical fibers.
In these two cases, the fibers which are coupled and utilized for the recording of the hologram of the holographic coupling lens are multimode fibers which are used in signal transmission via optical fibers. It has been observed that holographic couplers with multimode fibers provide low efficiency if the holograms are recorded by means of these fibers. This is due to a non-uniform distribution of light in the beams issuing from the fibers, a phenomenon called "speckling", which alters the interference image. The maximum efficiency is reduced by a factor of two to three according to the type of coupler.